[Bug 1954683] Re: grub is missing secure boot support for compressed kernels

[Julian Andres Klode]

** Also affects: grub2-unsigned (Ubuntu Impish)
   Importance: Undecided
       Status: New

** Also affects: grub2-unsigned (Ubuntu Focal)
   Importance: Undecided
       Status: New

** Also affects: grub2-unsigned (Ubuntu Jammy)
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1954683

Title:
  grub is missing secure boot support for compressed kernels

Status in grub2-unsigned package in Ubuntu:
  New
Status in grub2-unsigned source package in Focal:
  New
Status in grub2-unsigned source package in Impish:
  New
Status in grub2-unsigned source package in Jammy:
  New

Bug description:
  [Impact]
  Compressed kernels as we have on arm64 cause grub to fail in two ways:

  1. In all versions, grub-check-signatures will fail to verify the
  binaries using sbverify, complain about that in debconf, and then
  abort the installation/upgrade of grub-efi-arm64-signed

  2. In 2.06, the verifiers framework runs before any decompression,
  causing the kernels to fail verification, as it tries to verify the
  compressed data. In grub 2.04, we manually verified the file after we
  had opened it (hence after all filters).

  [Attack plan]
  1. Modify grub-check-signatures to optionally decompress kernels before passing them to sbverify
  2. Modify grub to either
     a) verify after decompress
     b) disable shim_lock verifier on arm64, and only use the rhboot

  We do not know if this is a long-term solution, we really should
  migrate back to kernels that are proper EFI executables themselves
  such that we can use standard EFI functions to run them as well.

  [Test plan]
  TBD

  [Where problems could occur]
  TBD

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